still spongy

i have 73 dodge dart power front disc brakes and 10" rear drums, changed all lines, hoses, calipers, wheel cylinders, rotors , drums , hardware, master cylinder, booster, pads and shoes using dot 3-4, flushed and bled, brakes work but not strong, when I check master cylinder you can see fluid move in front chamber(rear brake ) but little movement of fluid in back chamber(front brakes) proportioning valve is original to car and only thing haven't changed, fluid pumps through during bleeding , was wondering if valve could be bleeding off pressure internally? making spongy brakes, any thoughts would be appreciated, thanks